The invention relates to apparatus for atomic spectroscopy, and more specifically, the invention relates to a method and apparatus for concentrating a selected element of a sample in a furnace or electrothermal atomizer for atomic spectroscopy.
There exist several forms of atomic spectroscopy, each of which involves heating a sample to a selected temperature to prepare the sample for a spectroscopic measurement. For example, in atomic absorption spectroscopy, the procedure for preparing and measuring a trace element in a sample may include the steps of gently drying the sample by resistive heating in a furnace and pulse heating the furnace to an extremely high temperature (sometimes in the range of 3000.degree. C.) to atomize the sample. A light beam with selected frequency is then passed through the sample from which quantitative information is obtained by determining the extent of attenuation of the light caused by the presence of the trace material to be measured.
In theory, the measuring beam is attenuated only by atoms of the desired element, those being the atoms which absorb the same spectral lines. The analytical accuracy of this method, as well as the other methods, however, is often impaired when samples in a complex matrix are being analyzed. For example, included among the ways analytical accuracy may be effected are (a) loss of a desired element by prevolatilization as a molecular species; (b) formation of a molecular species containing the element caused by reactions in the gas phase involving the element; (c) uncorrectable molecular absorption or light scattering effects caused by particulate matter in the sample; and (d) non-uniform temperature along the furnace axis while the element is being vaporized.
Pre-volatilization involves the loss of the element at a temperature earlier than one would desire. One approach to avoid this result is to use a "platform" or "microboat", which is usually a piece of graphite laid within the furnace. The sample is deposited on the inserted member and the furnace is heated. Due to poor contact between the platform (or microboat) and the furnace wall, there exists a temperature lag in the heating rate of the inserted graphite and the wall. Thus, when the material is finally vaporized from the platform (or microboat), the inside temperature of the furnace is at a higher value and increases the probability that the element is released by thermal decomposition as a free atomic species.
The use of a platform or microboat, however, does not provide a means of controlling the temperature difference between the platform and the furnace. The microboat, therefore, does not provide a means for selectively vaporizing the element as needed.
With regard to gas phase molecular formation, the use of the platform or microboat assists in dissociating of interfering molecular species. More specifically, the increased gas phase temperature obtained at the time of vaporization promotes thermal dissociation. Because the platform does not provide for the selective vaporization of the sample, however, the extent of dissociation may not be complete. It would therefore be advantageous to be able to retain the element on the platform or microboat until the desired temperature is reached.
Background scatter of molecular absorption is caused by certain molecules absorbing the same wavelength as the element. Scatter can also be caused by the presence of particulate matter ("smoke"). Electronic correction systems using such means as a deuterium lamp or the Zeeman effect have been developed to correct these problems; but these systems are often not adequate and sometimes quite expensive.
Additionally, it is known that conventionally used furnaces often do not heat up uniformly along their length. As a result, the element to be measured often vaporizes and condenses at the cooler regions in the tube. While the use of the platform or microboat has permitted the heating of the furnace to a fairly high temperature before the element is released, it has been found that the temperature is still sometimes not uniform, and that the desired element condenses at the cooler regions. It would therefore be advantageous to be able to control the release of the element such that the sample is vaporized only when the furance is brought to a nearly uniform, high temperature.
Accordingly, it is desirable to provide a system which promotes thermal dissociation of the element to be measured from other molecular species, while at the same time minimizing loss by prevolatilization at a temperature earlier than desired.